1. Field of the Invention
The present invention is related to a method of treating a polymer film with a low temperature, e.g., 50.degree.-120.degree. Celsius, plasma to improve its physical properties, such as tensile strength.
2. Discussion of the Prior Art
Plastics films have attained a considerable degree of commercial success and are used in a wide variety of products. One such use is in the formation of bags where the film is subjected to considerable forces when in use. One film strength characteristic which is important, particularly in bag manufacture, is tensile strength. However, it has been found that tensile strength of a film can be adversely affected by the presence of low molecular weight polymer species which migrate to the film surface during melt crystallization of the polymer forming the film. These short chain molecules have fewer polymer entanglements than the underlying bulk material and therefore film failure is more easily initiated at a lower stress level than would be the case with the underlying film bulk material. A film failure initiated in the surface layer easily propagates into the underlying bulk material. In addition, the extrusion of molten polymer into a high flow of cooling air, such as occurs in blown film extrusion, causes surface embrittlement through oxidation. A brittle polymer surface coating on a more ductile substrate also reduces physical properties of a film, e.g., tensile strength, by increasing the ease of fracture initiation.
Several techniques, such as molecular orientation, uniaxial and biaxial stretching, etc. have been used to increase physical properties of a film. In general, however, these strength increasing techniques do not address the inherent weakness problems caused by a surface layer of a film which is more easily fractured than an underlying bulk material layer.